linux-kernel - Re: [PATCH v1] kernel/fork: only call untrack_pfn

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Message-ID: <0101d903-af59-478d-b0e6-af5ba6619eff@redhat.com>
Date: Thu, 24 Apr 2025 10:45:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
 Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...nel.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
 Rik van Riel <riel@...riel.com>, "H. Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v1] kernel/fork: only call untrack_pfn_clear() on VMAs
 duplicated for fork()

>> Probably the right way of attaching such metadata to a VMA would be
>> remembering it alongside the VMA in a very simple way.
>>
>> For example, when we perform a reservation we would allocate a refcounted
>> object and assign it to the VMA (pointer, xarray, whatever).
>>
>> Duplicating the VMA would increase the refcount. Freeing a VMA would
>> decrease the refcount.
>>
>> Once the refcount goes to zero, we undo the reservation and free the object.
>>
>> We would not adjust a reservation on partial VMA unmap (split + unmap A or
>> B), but I strongly assume that would just be fine as long as we undo the
>> reservation once the refcount goes to 0.
> 
> Yeah this is a really good idea actually, almost kinda what refcounts are
> for haha...
> 
> The problem is we talk about this idly here, but neither of us wants to
> actually write PAT code I'd say, so this may go nowhere. But maybe one of
> us will get so frustrated that we do this anyway but still...
> 
> Then again - actually, is this something you are planning to tackle?

I hate this much with that much passion that I'll give it a try for a 
couple of hours, as it might fix the other issues we are seeing.  So far 
it looks like it cleans up stuff *beautifully*. Even VM_PAT can go ... :)

... and I think we still have space in vm_area_struct without increasing 
it beyond 192 bytes.

-- 
Cheers,

David / dhildenb